(i) Technical Field
The present invention relates to an image reader, and a non-transitory computer readable medium storing image reading program.
(ii) Related Art
In general, in an image reader for reading a color image, a read body is irradiated with light, and reflected light or transmitted light is received by a light receiving element and is output as a signal of each color component. On an optical path to the light receiving element from the read body, an optical system such as a lens which forms an image on the light receiving element is disposed.
Light receiving characteristics of the light receiving element receives light of up to a wavelength longer than a wavelength band at the time of being observed by naked eyes, and when light of a wavelength which is invisible for the naked eyes is received and a signal is output, a color which is visible for the naked eyes and a color which is indicated by the signal output from the light receiving element are separated from each other. For example, spectral characteristics of a used light source are generated in various cases such as a case where light is emitted up to a wavelength band which is invisible for the naked eyes, or a case where a colorant used in an image forming apparatus absorbs light in a wavelength band which is visible for the naked eyes but reflects light in the wavelength band which is not visible for the naked eyes. In addition, spectral transmission characteristics of a blue (B) color and a green (G) color among color filters for performing a three-color separation on the light receiving element have characteristics of transmitting a 700 nm or greater infrared region, and thus there may be turbidity in a read color.
In a case of the filter for eliminating the infrared light by the interference film, the spectral characteristics are changed according to an incident angle. Then, light passing through an optical axis, and light passing through a position separated from the optical axis have different spectral characteristics, and thus there is a difference in the read color. In addition, recently, a white color light-emitting diode (hereinafter, referred to as a “white color LED”) is used as the light source. Emitted light of the white color LED has spectral characteristics different from the xenon lamp, and a change of the spectral characteristics according to the incident angle may affect the read color.
In addition, there is a trend in which a spectral frequency band of an R component extends to a short wavelength side according to needs for high sensitivity of the light receiving element. Accordingly, a half value wavelength of the R component is shorter than a half value wavelength of a red colorant, and for example, there is a trend in which a spectral distribution of the R component at the time of reading an achromatic color and a spectral distribution of the R component at the time of reading a red color are separated from each other.
FIGS. 5A and 5B are explanatory views of an example of the spectral distributions of the R component at the time of reading the achromatic color and the red color. In FIG. 5A, as the spectral distribution of the R component at the time of reading the achromatic color, a solid line indicates the spectral distribution in the optical axis, and a broken line indicates the spectral distribution in the position separated from the optical axis. In addition, in FIG. 5B, as the spectral distribution of the R component at the time of reading the red color, a solid line indicates the spectral distribution in the optical axis, and a broken line indicates the spectral distribution in the position separated from the optical axis. In any case, a portion which is indicated by a hatched line is a portion in which the spectral characteristics are changed according to the incident angle by the filter for eliminating the infrared light by the interference film. An output value of a signal of the R component corresponds to an area of the spectral distribution thereof.
In general, a shading correction for correcting a limb darkening of the lens or the like is performed based on an output value of each color component at the time of reading the achromatic color. By an output value W (an area of the spectral distribution) at the time of reading the achromatic color illustrated in FIG. 5A, an output value R (an area of the spectral distribution) at the time of reading the red color illustrated in FIG. 5B is normalized, and R/W is considered. An output value of the R component at the time of reading the achromatic color in the optical axis is set to Wc, an output value of the R component at the time of reading the red color is set to Rc, and Rc/Wc is considered as a normalized value. In addition, an output value of the R component at the time of reading the achromatic color in the position separated from the optical axis is set to Wt, an output value of the R component at the time of reading the red color is set to Rt, and Rt/ Wt is considered as a normalized value. Here, when a change of the value according to a change of the spectral characteristics of the filter which is indicated by the hatched line in FIGS. 5A and 5B is set to S1, and it is assumed that the changed values S1 at the time of reading the achromatic color and at the time of reading the red color are identical to each other, Wt may be approximated by (Wc−S1), and Rt may be approximated by (Rc−S1). Here, when Rc/Wc and (Rc−S1)/(Wc−S1) are compared to each other, an influence of the changed value S1 decreases as a value of a fraction thereof becomes closer to 1, but a difference between the values of both of the fractions according to the changed value S1 increases as the value of the fraction becomes smaller. As described above, the value of the fraction decreases along with the increasing spectral sensitivity width of the light receiving element, and thus the influence according to the change of the spectral characteristics of the filter appears. A decrease in Rc/Wc indicates a decrease in color saturation of the read value of the red color. Furthermore, a color turbidity correction amplifies the difference of color saturation, and thus a read color saturation characteristic difference according to the incident angle is emphasized.